This invention relates to methods and apparatus for detecting, locating, quantifying and measuring the depth of impact caused delaminations and cracking in laminates. Such defects are particularly difficult to detect because a microscopic crack which is visually indistinguishable from other microscopic cracks on the laminate surface may be the only evidence of massive surface parallel delamination or surface perpendicular branched cracking within the laminate. The invention method and apparatus use the microscopic surface breaking cracks that are associated with impact caused subsurface delamination and cracking as a means of communicating between the surface delaminations and cracks and the laminate surface.
Laminates, often comprised of cloths of kevlar, nylon, graphite, composites or other materials, embedded with expoxy-catalyst mixtures or other bonding materials and pressed together are used in a wide variety of applications including many in which the structural integrity of the laminate is critical. Examples of such uses are containers such as filament wound kevlar-epoxy rocketmotor cases and fuel tanks. Impact damage to such laminate structures is difficult to detect, however. Delamination and cracking due to an impact can be below the laminate surface with no visible indication thereof on the surface. Moreover the subsurface damage may comprise either or both branching, surface perpendicular cracking or surface parallel delamination. The inhomogenous and layered nature of the laminate makes inspection by usual nondestructive testing methods impractical. Two-sided access is often impossible due to the geometry of particular laminate structures. Traditional sonic and ultrasonic pulse-echo NDT methods are unreliable in all but the thinnest laminate structures due to the interference caused by the numerous laminae, small air pockets and inhomogeneties inherent in laminates. Other methods including those using electrified particles, electrical capacitance, and acoustics have also been found to be inappropriate for testing hard to inspect laminates. Radiography is impractical due to the one-sided access only.
In spite of a long existing commercial need for a method and apparatus useable for testing laminate structures and numerous attempts to use known methods and apparatus for that purpose, none have solved this need until disclosure of the instant invention.